This invention relates to a communication network and a method of allocating resource therefor, and in particularly to a CDMA cellular radio communication network and a method therefor.
In a cellular communication system each of the user terminals (typically mobile stations) communicates with typically a fixed base station (base station transceiverxe2x80x94BTS). Communication from the user terminal to the base station is known as uplink and communication from the base station to the user terminal is known as downlink. The total coverage area of the system is divided into a number of separate cells, each predominantly covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells. FIG. 1 illustrates a cellular communication system 100. In the system, a base station 101 communicates with a number of user terminals 103 over radio channels 105. In the cellular system, the base station 101 covers users within a certain geographical area 107, whereas other geographical areas 109, 111 are covered by other base stations 113, 115.
As a user terminal moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the user terminal and the base station of the first cell, to being between the user terminal and the base station of the second cell. This is known as a handover. Specifically, some cells may lie completely within the coverage of other larger cells.
All base stations are interconnected by a fixed network. This fixed network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network. A call from a user terminal is routed through the fixed network to the destination specific for this call. If the call is between two user terminals of the same communication system the call will be routed through the fixed network to the base station of the cell in which the other user terminal currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a user terminal and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.
A cellular mobile communication system is allocated a frequency spectrum for the radio communication between the user terminals and the base stations. This spectrum must be shared between all user terminals simultaneously using the system.
One method of sharing this spectrum is by a technique known as Code Division Multiple Access (CDMA). In a Direct Sequence CDMA (DS-CDMA) communication system, the signals are, prior to being transmitted, multiplied by a high rate code whereby the signal is spread over a larger frequency spectrum. A narrowband signal is thus spread and transmitted as a wideband signal. At the receiver-the original narrowband signal is regenerated by multiplication of the received signal with the same code. A signal spread by use of a different code will at the receiver not be de-spread but will remain a wide band signal. In the receiver the majority of interference caused by interfering signals received in the same frequency spectrum as the wanted signal can thus be removed by filtering. Consequently a plurality of user terminals can be accommodated in the same wideband spectrum by allocating different codes for different user terminals. Codes are chosen to minimise the interference caused between user terminals typically by choosing orthogonal codes when possible. A further description of CDMA communication systems can be found in xe2x80x98Spread Spectrum CDMA Systems for Wireless Communicationsxe2x80x99, Glisic and Vucetic, Artech house Publishers, 1997, ISBN 0-89006-858-5. Examples of CDMA cellular communication systems are IS 95 standardised in North America and the Universal Mobile Telecommunication System (UMTS) currently under standardisation in Europe.
Typically, CDMA communication systems use a handover technique known as soft hand over. In a soft handover, a user terminal is simultaneously served by more than one base station. FIG. 2 illustrates the situation where a user terminal 201 is in soft handover with two base stations 203, 205 each covering a cell 207, 209. In the uplink the signal received at each base station 203, 205 is communicated to a controller 211 where the signals are combined and the received data is recovered. In the downlink the data to be transmitted to the user terminal is communicated from the controller to both base stations 203, 205 and both base stations simultaneously transmit the data to the user terminal 201. The user terminal 201 combines the two signals and demodulates the signal.
The quality of the radio communication between the user terminal and the base station is determined by the signal to noise level of the signals where the noise includes both thermal noise and interference noise. Other base stations and user terminals generate interference, which increases the noise level and thus reduces the quality. In order to attain an acceptable quality level the interference must thus be kept sufficiently low. A major technique for interference reduction in CDMA system is use of power control whereby the transmitted power of each user terminal and base station is maintained at the minimum level required for the signal to be received at an acceptable quality. Uplink power control can be implemented by the base station measuring the received signal quality and transmitting power up information to the user terminal when the signal quality is below an acceptable level, and power down information when the signal quality is above this level. Similarly, downlink power control can be implemented by the user terminal transmitting power up or power down information depending on the signal quality of the signal received at the user terminal.
In a communication system such as cellular communication systems, efficient resource control is essential for achieving the highest performance of the communication system. The communication thus comprises means for controlling the transmission of data services, allocating resources to different users, scheduling the transmission of data, providing error checks, determining serving base station etc.
The procedures and routines required for controlling data transmission in communication systems are typically considered in the frame of a layered logical structure where lower layers provide functionality to higher layers. The lowest layer is the physical layer which is responsible for communicating data bits over the radio communication links between base stations and user terminals. The physical layer thus provides a number of bit communication services to the next layer which is the MAC (Medium Access Control) layer. The MAC layer provides functionality for allocation of available resource to different users and for scheduling data transmissions to meet required quality of service. Above the MAC, the RLC (Radio Link Control) protocol allows retransmissions of errored data across the radio link. The Radio Resource Control sits above the RLC and covers functions including call admission, configuration of other entities (like the MAC and physical layer) and management of radio resources across multiple cells.
The resource allocation and data scheduling is a very complicated process where allocation in one cell may impact the performance in a different cell. This is specifically problematic for a communication network such as a CDMA network operating in soft handover where a specific user terminal may be served by several parallel communication links from different base stations. Hence, ideally, a resource allocation should consider conditions and resource requirements in all cells and perform a global optimisation in response to this. However, due to the complexity of such a resource allocation this is impractical for all but the simplest communication networks.
Conversely simply considering resource allocation based on a single cell is inefficient, if at all possible. As a specific example, if a user terminal receiving short data bursts is in soft handover with two cells, the data bursts are required to be transmitted substantially simultaneously from the two base stations. Allocation of resource independently in these two base stations is thus not possible. As there may simultaneously be many user terminals in soft handover and as the number and parameters for these soft handovers change frequently it is not feasible to define certain base stations as master base stations controlling the resource allocation of other slave base stations. This would lead to conflicts where for some soft handover scenarios the same base station would be required to be both a master and slave base station.
A low complexity system for allocating resource in a communication network having a plurality of communication links serving the same user terminal is thus advantageous.
The current invention seeks to provide a communication network having a low complexity system for allocating resource in a communication network having a plurality of communication links serving the same user terminal.
According to the present invention, there is provided a communication network comprising first resource allocating means for allocating resource in a first communication channel; second resource allocating means for allocating resource in a second communication channel; first communication means for providing a communication link for the first communication channel to at least one user terminal; second communication means for providing a communication link for the second communication channel to at least one user terminal; a first user terminal using both communication links for communication; means for determining an associated resource allocating means from said first or second resource allocating means for said first user terminal; and wherein said associated resource allocating means performs a first allocation of resource to said first user terminal without consideration of resource allocation in other resource allocating means and the resource allocating means not being associated resource allocation means subsequently performs a resource allocation to the first user terminal in response to the first resource allocation of the associated resource allocation means.
Preferably, the resource allocating means not being associated resource allocation means perform an initial resource allocation not considering resource allocation for the first user terminal, and the resource allocation to the first user terminal in response to the first resource allocation of the associated resource allocation means consist in modifying this initial resource allocation.
According to one feature of the invention the communication network is a CDMA cellular communication network and the first user terminal is in soft handover.
According to a different aspect of the invention, a method of allocating resource in a communication network having first resource allocating means for allocating resource in a first communication channel; second resource allocating means for allocating resource in a second communication channel; first communication means for providing a communication link for the first communication channel to at least one user terminal; second communication means for providing a communication link for the second communication channel to at least one user terminal; and a first user terminal using both communication links for communication; the method comprising the steps of: determining an associated resource allocating means from said first or second resource allocating means for said first user terminal; in said associated resource allocating means performing a first allocation of resource to said first user terminal without consideration of resource allocation in other resource allocating means communicating the first allocation of resource to the resource allocating means not being associated resource allocation means; and in the resource allocating means not being associated resource allocation means performing a resource allocation to the first user terminal in response to the first resource allocation of the associated resource allocation means.
The invention thus provide low complexity, fair and efficient means of scheduling resource to allow the many benefits of soft handover to be realised.